1. Field of the Invention
The present invention relates to a method of applying a composite polyvinylchloride plastisol coating to a metal sheet to produce a coating having high hardness and excellent impression resistance, bleed resistance and formability. The invention also relates to polyvinylchloride plastisols used in such method.
2. Description of the Prior Art
Polyvinylchloride plastisol coatings for metal sheets are widely applied for various uses because of the coatings' superior formability, corrosion resistance, weatherability and capability of producing surface designs by embossing. However, conventional polyvinylchloride plastisol coatings for metal sheets have an inferior impression resistance and bleed resistance because of the softness of the coating. Therefore, the conventional polyvinylchloride plastisol coatings are easily deformed when the coated metal sheets are piled up in stacks, causing impressions to be formed in the embossed patterns of the coatings as well as bleeding of the coatings.
To improve upon the above-mentioned drawbacks of conventional polyvinylchloride plastisol coatings, the polyvinylchloride powders, plasticizers and diluents incorporated in the polyvinylchloride plastisols have been examined to obtain a harder coating, however, the improvements have been limited.
In the prior art, there are known methods relating to improvements of the above-mentioned drawbacks of polyvinylchloride plastisol coatings, for example, Japanese Patent Application Laid-Open No. Sho. 54-50549 and Japanese Patent Application Laid-Open No. Sho. 63-185630. However, the present invention is different from these inventions with regard to the components of the plastisol.
Japanese Patent Application Laid-Open No. Sho. 54-50549 relates to a polyvinylchloride plastisol coating for metal sheets having high hardness and superior bleed resistance, stain resistance and solvent resistance. The reference discloses a composite plastisol consisting of a conventional polyvinylchloride powder, reactive plasticizer and conventional plasticizer which is coated on a metal sheet and heated, and, in certain instances, is irradiated by ultraviolet rays. However, the hardness, impression resistance and bleed resistance of the coating are not completely satisfactory.
Japanese Patent Application Laid-Open No. Sho. 63-185630 also relates to a polyvinylchloride plastisol coating for metal sheets. This reference discloses a composite plastisol consisting of a specific polyvinylchloride powder and an acrylic resin powder which is coated on a metal sheet and heated. The present invention is different from this invention with regard to the materials used in the plastisol and with regard to the hardness, impression resistance and bleed resistance of the coating.